This invention relates to a cryostat in which cooling is achieved by the isenthalpic expansion of a high-pressure gas through a Joule-Thomson orifice, and, more particularly, to a two-stage cryostat having a gas flow management system for achieving rapid cooldown.
Many types of devices, such as infrared detectors, are operated at very low temperatures, as for example 100K or less. In some cases, low temperature operation is required because physical or chemical processes of interest occur only at low temperature or are more pronounced at low temperature, and in other cases because some types of electrical-thermal noise are reduced at low temperature. An approach to cool the device to low temperature is therefore required.
The simplest and most direct approach to cooling a device to a low operating temperature is to bring the device into thermal contact with a bath of liquid gas whose normal boiling temperature is approximately the desired operating temperature. This liquid contacting bath ensures that the temperature of the device will not exceed the boiling temperature of the liquefied gas.
While the liquid contacting bath approach is preferred for laboratory and other stationary cooling requirements, the cooling of small devices in mobile applications, or other situations that make the use of stored liquid coolants difficult, requires another approach. For example, it may not be possible to provide liquefied gas to a device operated in a remote site, or in space. Also, it may be inconvenient or impossible to store liquefied gases for long periods of time, or periodically service the store of liquefied gas.
Various approaches have been developed to cool devices to a low operating temperature, without using stored liquefied gas as a contacting bath coolant. For example, gas expansion coolers expand compressed gas through a Joule-Thomson orifice, thereby cooling and partially liquefying the gas and resulting in absorption of heat from the device to be cooled, the cooling load. Several types of thermoelectric devices and closed cycle mechanical gas refrigerators can also be used.
The various cooling approaches that do not require a stored liquefied gas are operable and useful in a range of situations. However, they all have the shortcoming that they cannot achieve very rapid cooling of the cooling loads demanded by many systems. The fastest cooldown times are achievable with a Joule-Thomson gas expansion cryostat, which is known to have the capability of cooling very small thermal load masses with removable enthalpy values of tens of Joules to approximately 120 K. within a few seconds. However, when the thermal mass load is significantly larger and when lower cold temperature is required, the conventional Joule-Thomson cryostat is inadequate. For example, a conventional Joule-Thomson gas expansion cryostat may require 30 seconds and typically more than a minute to cool a device from ambient temperature to a temperature of 80 K., removing about 250 Joules in the cooling process. This cooling rate is simply too slow for some mobile applications, where cooling times of 5-20 seconds may be required. Thus, although many cooling devices that do not require stored liquefied gas can cool to low temperature, available systems achieve this cooling rather slowly.
Additionally, some specialized devices and cooling systems have unique packaging and space requirements. For example, an infrared heat seeking detector in the nose of a missile must be securely supported and rapidly cooled upon demand, but the overall size and weight of the cooling system is severely limited by the overall system constraints.
There is a need for a cooling apparatus that does not require stored liquefied gas, and that achieves very rapid cooling of large thermal mass loads to temperatures of 80 K. or less. The size and weight of the cooling apparatus, including the hardware and any stored consumables that may be required, must be as small as possible. The present invention fulfills this need, and further provides related advantages.